Electrochemical sensor element with a porous reference gas accumulator

ABSTRACT

An electrochemical sensor element, in particular for determining the oxygen level in gas mixtures, includes at least one measuring electrode exposed to a measured gas, at least one reference electrode exposed to a reference gas, at least one heating device, and one reference gas channel, through which the reference gas can be supplied to the reference electrode. The reference electrode is connected to the reference gas via a volume provided with pores. The volume is formed in a layer between the reference gas channel and the reference electrode.

FIELD OF THE INVENTION

The present invention relates to an electrochemical sensor element, inparticular for determining the oxygen level in gas mixtures.

BACKGROUND INFORMATION

Sensor elements are known. They are designed as planar sensor elements,which have, on a solid electrolyte designed as a support, a firstelectrode exposed to the measured gas and a second gas exposed to areference gas. Furthermore, an electrical resistance heater is embeddedin the support. A reference gas, which is in most cases made up ofatmospheric air, is supplied to the reference electrode via a referencegas channel integrated in the support. At the same time, the referencechannel forms a gas chamber having a bottom surface matching thereference electrode in the reference electrode area, so that sufficientoxygen may reach the reference electrode.

It is known from European Patent No. 125069 that the width of thereference gas channel can be adapted to match the width of the electrodeover its entire length for this purpose, or two reference gas channels,with one electrode arranged in each, can run in a layer plane parallelto one another, with the two electrodes connected together, forming thereference electrode. The disadvantage of a wide reference gas channel ora reference gas channel made up of two adjacent parts is that one partof the heating coil of the resistance heater element is always in thearea of the perpendicular projection of the reference gas channel. Thisresults in overheating of the solid electrolyte in the area of thereference gas channel. In addition, a wide reference gas channelprovides poor heat transfer between the resistance heating element andthe electrodes.

The method described in German Patent Application No. 19609323 in whichthe reference gas channel is branched in the area of the heating device,offers a possible remedy. However, in this case the reference electrodesmust also be branched.

SUMMARY OF THE INVENTION

The sensor element according to the present invention has the advantagethat it allows improved heat transfer between the electrodes and theresistance heating element, resulting in uniform heat distribution. Theporous layer also helps relieve mechanical stresses that occur at theedges where the reference gas channel and the adjacent solid electrolytefilm meet, and which may result in stress cracks in the ceramic support.In bridging a wide reference gas channel, the solid electrolyte film isbent, which results in additional mechanical stresses. Using the narrowreference gas channel, excessive bending of the adjacent solidelectrolyte film is avoided. Furthermore, due to the large-surfacecontact of the reference electrode with the adjacent porous layer,better adhesion of the latter is achieved, since the reference electroderemains pressed between the adjacent films during lamination. This isalso true for the lead to the reference electrode, with its resistancealso being thereby reduced.

It should be emphasized that the reference gas channel may have aslightly widened handgrip shape in the area of the reference electrode.This allows oxygen exchange to be improved, in particular in the case oflow pore volumes. The effect of the reference atmosphere can beintensified by adding an oxygen-storing material, for example, CeO₂, tothe porous layer. This can be achieved by impregnating the porous layeror the porous electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section through the sensitive part of a sensorelement according to the present invention.

FIG. 2 shows a longitudinal section through the sensor element alongline II—II of FIG. 1 according to a first embodiment.

FIG. 3 shows a longitudinal section through the sensor element alongline II—II of FIG. 1 according to a second embodiment.

DETAILED DESCRIPTION

FIG. 1 shows a cross section through the measured gas side of a sensorelement. The sensor element is a component of a gas sensor (not shown)and is secured in a housing of the sensor and its sensitive section isexposed to a gas to be measured. The sensor element is made of a ceramicsupport 10 having a planar layer structure with a first solidelectrolyte film 11, a second solid electrolyte film 12, and a thirdsolid electrolyte film 13 superimposed on one another. An outer largesurface of first solid electrolyte film 11 has a measuring electrode 15and its inner large surface has a reference electrode 17. Measuringelectrode 15 is covered with a porous protective layer 19. Adjacent tofirst solid electrolyte film 11 is second solid electrolyte film 12,which has a narrow hollow cavity forming reference gas channel 21 in themiddle. Between second solid electrolyte film 12 and third solidelectrolyte film 13 an electrical resistance heating element 23 isarranged between two electrical insulating layers 25. Since electricalinsulating layers 25 are porous so they can absorb mechanical stressesdue to the different heat expansion coefficients of the materials used,a gas-tight solid electrolyte frame 27 is placed around insulating layer25. Electrical resistance heating element 23 is designed as a heatingcoil on the sensitive section of the sensor element.

On one narrow side of ceramic support 10, reference channel 21 has areference gas opening 29 and runs approximately to the opposite end faceof ceramic support 10, where reference gas channel 21 is closed. In theembodiment of FIG. 2, reference gas channel 21 has a uniform, forexample, rectangular, cross section over its entire length. For example,the width of unsintered reference gas channel 21 is 0.4 mm to 0.8 mm,preferably 0.6 mm. The height of reference gas channel 21 is equal tothe thickness of sintered solid electrolyte film 12, for example, 0.4mm. In the embodiment of FIG. 3, reference gas channel 21 has a slightlywidened section 31 in the area of reference electrode 17, so thatreference gas channel 21 has a handgrip shape overall when viewed fromabove. Reference gas channel 21 may also branch off in the area ofreference electrode 17.

Reference electrode 17, which has a flat shape in the plane of ceramicsupport 10, is covered with a porous layer 33 according to a firstembodiment. Porous layer 33, which is represented by a dotted surface inFIGS. 2 and 3, is embedded between reference electrode 17 and theadjacent second solid electrolyte film 12. First solid electrolyte film11 has a depression, for example, on whose bottom reference electrode 17is placed with the porous layer filling the depression over referenceelectrode 17. Thus porous layer 33 spans reference gas channel 21 inthis area after films 11, 12, 13 have been laminated together. Thereference gas penetrating via reference gas opening 29 then diffuses viaporous layer 33 to reference electrode 17 positioned thereon. Thethickness of the porous layer is 5 μm to 200 μm, preferably 20 μm to 50μm.

In another embodiment for performing gas exchange with the referencegas, reference electrode 17 itself has a porous design. Furthermore, anembodiment may use one porous layer and one porous reference electrode.A suitable pore volume is required in order to form an appropriatereference gas chamber. This is achieved through the thickness of porouslayer 33 and/or of porous reference electrode 17.

Good oxygen exchange can be achieved at reference electrode 17 by addingan oxygen-storing material, for example, CeO₂, to porous layer 33 and/orporous reference electrode 17. The oxygen-storing material can be addedby impregnating porous layer 33 and/or porous reference electrode 17.

If the sensor element is operated as a concentration cell, referenceelectrode 17 can be supplied with sufficient oxygen by applying anelectric voltage to measuring electrode 15 and reference electrode 17.Thus an oxygen pumping effect is achieved in that oxygen is pumped frommeasuring electrode 15 to reference electrode 17. An additional pumpedinternal oxygen reference is thus formed on reference electrode 17.

What is claimed is:
 1. An electrochemical sensor element for determiningan oxygen level in a gas mixture, comprising: at least one measuringelectrode exposed to a measured gas; at least one reference electrodeexposed to a reference gas having an upper side proximate to themeasuring electrode and a lower side distal to the measuring electrode;a porous layer containing a volume provided with pores, the layer beingarranged adjacent to the lower side of the reference electrode such thatthe reference electrode is exposed to the reference gas via the layer;and a reference gas channel adjacent to the porous layer through whichthe reference gas is supplied to the reference electrode via the porouslayer.
 2. The sensor element according to claim 1, the layer defining afirst volume and the reference gas channel defining a second volume. 3.The sensor element according to claim 1, further comprising anoxygen-storing material added to at least one of the layer and thereference electrode.
 4. The sensor element according to claim 3, whereinthe oxygen-storing material includes CeO₂.
 5. The sensor elementaccording to claim 1, wherein the measuring electrode and the referenceelectrode are connected as a concentration cell, an electrical voltagebeing applied to the measuring electrode and the reference electrode,resulting in an oxygen pumping effect from the measuring electrode tothe reference electrode, a pumped internal oxygen reference being formedin the volume provided with pores.
 6. An electrochemical sensor elementfor determining an oxygen level in a gas mixture, comprising: at leastone measuring electrode exposed to a measured gas; at least onereference electrode exposed to a reference gas having an upper sideproximate to the measuring electrode and a lower side distal to themeasuring electrode; a layer containing a volume provided with pores,the layer being arranged adjacent to the lower side of the referenceelectrode such that the reference electrode is exposed to the referencegas via the layer; and a reference gas channel adjacent to the layerthrough which the reference gas is supplied to the reference electrodevia the layer, the reference electrode oriented with a longitudinal axisparallel to a longitudinal axis of the reference gas channel, a width ofthe reference electrode perpendicular to the longitudinal axis of thereference electrode, a width of the reference gas channel perpendicularto the longitudinal axis of the reference gas channel, wherein the widthof the reference electrode is greater than the width of the referencegas channel.
 7. An electrochemical sensor element for determining anoxygen level in a gas mixture, comprising: at least one measuringelectrode exposed to a measured gas; at least one reference electrodeexposed to a reference gas having an upper side proximate to themeasuring electrode and a lower side distal to the measuring electrode;at least one heating device; a layer containing a volume provided withpores, the layer being arranged adjacent to the lower side of thereference electrode such that the reference electrode is exposed to thereference gas via the layer; and a reference gas channel adjacent to thelayer through which the reference gas is supplied to the referenceelectrode via the layer, wherein the reference gas channel is formed asa hollow space containing no porous material.
 8. An electrochemicalsensor element for determining an oxygen level in a gas mixture,comprising: at least one measuring electrode exposed to a measured gas;at least one porous reference electrode exposed to a reference gashaving an upper side proximate to the measuring electrode and a lowerside distal to the measuring electrode; at least one heating device; alayer containing a volume provided with pores, the layer being arrangedadjacent to the lower side of the reference electrode such that thereference electrode is exposed to the reference gas via the layer; and areference gas channel adjacent to the layer through which the referencegas is supplied to the porous reference electrode via the layer.
 9. Anelectrochemical sensor element for determining an oxygen level in a gasmixture, comprising: at least one measuring electrode exposed to ameasured gas; at least one reference electrode exposed to a referencegas having an upper side proximate to the measuring electrode and alower side distal to the measuring electrode; a porous layer containinga volume provided with pores, the layer being arranged adjacent to thelower side of the reference electrode such that the reference electrodeis exposed to the reference gas via the layer, the layer defining afirst volume; and a reference gas channel adjacent to the porous layerthrough which the reference gas is supplied to the reference electrodevia the porous layer, the reference gas channel defining a secondvolume, the second volume forming a hollow space containing no porousmaterial.